1. Field of the Invention
The present invention relates to a method of preventing colors from being mixed in a two-color image forming apparatus wherein a two-color image is formed in a single step of color image formation.
2. Description of the Related Art
A two-color image forming apparatus of the type in which a two-color image is formed in a single step of color image formation is well known. In this type of image forming apparatus, two separate latent electrostatic image forming, and developing stages are used for forming an image having two colors. During latent image forming/developing, the toner used for forming a first toner image in the first latent image forming/developing stage is inevitably mixed with the toner of the other color for forming a second toner image. As a result, the second toner image contains a mixture of the colors of those toners. This will be described in more detail with reference to FIG. 1.
FIG. 1 is a sectional view showing the construction of a part of a conventional two-color image forming apparatus. As shown, two latent image forming/developing stages are disposed around a photoreceptor 1 as a drum. The first latent image forming/developing stage A, which forms a first latent electrostatic image and develops it into a first toner image, includes a first charger 2 and a first developing unit 4. A first laser beam 3 containing visual information, such as characters and graphic information, is applied to the charged surface of the photoreceptor 1.
The second latent image forming/developing stage B, which forms a second latent electrostatic image and develops it into a second toner image of a second color different from the first color, is disposed downstream of the first latent image forming/developing stage. This stage includes a second charger 5, and a second developing unit 7. A second laser beam 6 containing a visual information is applied to the charged surface of the photoreceptor 1, after it leaves the second charger 5.
A transfer unit 8 is disposed downstream of the second latent image forming/developing stage. The transfer unit 8 transfers the first and second toner images, which were formed on the surface of the photoreceptor 1 in the first and second latent image forming/developing stages, onto a recording paper 9.
In operation, a first toner image is formed on the surface of the photoreceptor 1 through a process consisting of the following steps:
(1) uniformly charging the surface of the photoreceptor 1 using the first charger 2; PA1 (2) forming a first latent electrostatic image on the surface of the photoreceptor 1 with the first laser beam 3; and PA1 (3) developing the first latent electrostatic image with the first developing unit 4. PA1 (1) uniformly charging the surface of the photoreceptor 1 using the second charger 5; PA1 (2) forming a second latent electrostatic image on the surface of the photoreceptor 1 with the second laser beam 6; and PA1 (3) developing the second latent electrostatic image with the second developing unit 7.
Similarly, a second toner image is formed on the surface of the photoreceptor 1 through a second process consisting of the following steps:
The first and second toner images thus formed are transferred onto the recording paper 9 by the transfer unit 8.
In the above-described known apparatus for forming an image in two different colors, during the process of forming and developing the first and second latent electrostatic images, the toner forming the first toner image on the surface of the photoreceptor 1 is inevitably partially scraped off the surface of the photoreceptor 1, and enters into the second developing unit 7. In the second developing unit 7, the toner of the first toner image is mixed into the toner of the second toner image, so that the second toner image contains a mixture of the first color (of the first toner) and the second color (of the second toner).
This mixture of the toners also varies the charge properties, e.g., the amount of charge and the polarity of charge, of the second toner. Accordingly, the quality of the resultant reproduced picture is deteriorated. Also, the lifetime of the developer is reduced.